We're lucky to have some nice math kits with manipulatives that we pull from for centers. I also have made tons of math file folder games that the students enjoy. Make up simple games with dice, number tiles, bingo boards, unifix cubes; foldables for place value; dominoes and decks of cards for adding/subtracting/multiplying. It doesn't have to be fancy, just make sure you explain each center well and have some kind of recording sheet for their accountability.

I teach second grade, but these centers would work for third too, I think.
-We had kids measuring area using Cheese-Its. First I made up a sheet with shapes on it. The students estimated first, then measured using the crackers. Best part of all - eating.
-We also made a patterning center using skittles. We gave the kids each their own bag of Skittles and they made up their own patterns and recorded them on paper. Then, of course, eat!
-We also estimated and measured length of string licorice at a center. Then graphed the lengths of the different pieces on a bar graph.
-We got artistic by using strips of paper, dividing a piece of paper into thirds and gluing the strips to make horizontal, vertical, and diagonal lines.
The kids loved all these centers!

A good website that i know that could be used in a math center or possibly other centers as well. Sheppard Software.com is a great website. I've used the fraction shooting on there as a center in math when I taught my students about fractions. But the website has many different types of activites that the students can do during different content areas or in math.